As the wireless communication technology progresses, various kinds of applications and services are also devised. IEEE 1609 protocol has the characteristics of alternating switching between control channel (CCH) and service channel (SCH), and repeated transmission of WAVE service advertisement (WSA) so as to facilitate the application of the wireless communication in the vehicles. This protocol includes a protocol group, with each part respectively processing different layers of the Open System Interconnection (OSI) model, and in combination with IEEE802.11p physical layer for communication. For example, IEEE 1609.1 is the resource manager, IEEE 1609.2 is to handle the security services of the application and management message, IEEE 1609.3 is to handle the networking services and IEEE 1609.4 is to handle the multi-channel operation.
In IEEE 1609 protocol, the frequency band (5.8 GHz) is divided into 7 groups of channels, as shown in the channel frequency allocation of FIG. 1A, where channel 178 is the control channel and the remaining six channels are service channels. IEEE 1609 protocol specifies a device as a system having at least a single antenna, and divides the time sequence into control interval (CCH interval) and service interval (SCH interval). The bottom layer of IEEE 1609 uses channels of different frequencies simultaneously to transmit difference services, and specifies the single control channel and the multiple service channels to switch alternatingly. All the devices will switch alternatingly and regularly in the control interval and service interval in a synchronous manner at the start of every coordinated universal time (UTC) second, and switch alternatingly the channel frequency of own antenna according to the respective services, as shown in FIG. 1B.
The control interval is for transmitting emergency information, security requirement information, Wireless Access in Vehicular Environments (WAVE) service advertisement (WSA) provided by service providers and WAVE short message protocol (WSMP) of high priority. The service interval is for short message protocol of general priority to exchange information and for service user and service provider to exchange information required for the service. All the supported data formats may use designated Medium Access Control (MAC) for unicast or broadcast.
IEEE 1609 protocol also provides the flexibility of a plurality of service channel accesses. As shown in the four service channel access modes of FIG. 2, the area with slanted pattern is using the control channel and the clear area is using the service channel. Hence, the continuous access mode allows the device to continue the control channel without the channel coordination when no service channel is available. Alternating access mode allows the device to alternatingly use the control channel and the service channel. The immediate access mode allows the device to switch to service channel immediately even when in CCH interval, marked as 231. Extended access mode allows the device to remain in service channel to extend the service channel without switching to control channel after the service interval ends, marked as 232.
Because of the restriction of communication physical layer, a single antenna can only monitor a channel at a time. Hence, the device with a single antenna may only select services provided by a single channel. When the required services are on two or more channels, only one service channel may be selected from the two or more channels to transmit the service and the rest service channels must be given up.
As shown in FIG. 3, U.S. Patent Publication No. 2008/0232309 discloses an adaptive queuing technology for discontinuous channel. In the interval of service channel 310, the technology adjusts the transmission order of data segments transmitted in device service channel transmission queue 320, such as, larger transmission data segment 322 and a plurality of smaller transmission data segments 321, to avoid larger data segment 322 from delay due to channel switching, thereby increasing the utilization of the channel.
U.S. Patent Publication No. 2008/0232433 discloses a short distance communication technology by using an adaptive channel interval to increase or decrease the interval between switches for improving the utilization of the channel. In one embodiment of the technology based on IEEE 1609, the channels are switched for every fixed interval to obtain emergency information and other service information simultaneously. The time to switch channel is adaptively adjusted for different user scenarios, such as, when the need for emergency information is high, the control interval is extended and the next service interval decreases equally.